1. Field of the Invention
This invention relates to semipolar plane III-nitride semiconductor-based laser diodes (LDs) with AlGaN barriers and a superlattice separate confinement heterostructure (SCH), and methods of fabrication thereof.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., Ref. [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Wurtzite (Al, Ga, In)N laser diodes (LDs) are one of the promising candidates for green laser applications. Since the first c-plane GaN-based Laser Diode (LD) was demonstrated by Nakamura et al. [1], there have been significant developments towards long wavelength LDs. Recently, the longest lasing wavelength of c-plane LDs reached 532 nm under pulsed operation [2]. Despite successful demonstration of the green LDs, the devices grown on the c-plane suffer from the Quantum Confined Stark Effect (QCSE) due to the large polarization-related electric fields, which cause lower internal quantum efficiencies due to the spatial separation of the electron and hole wave functions in the quantum wells [3]. This may also cause higher operation voltage, which results in the small wall plug efficiency [2]. Nonpolar and semipolar GaN-based devices are also promising for longer wavelength LDs because they exhibit no or very little QCSE [4-7]. Higher gain for LDs grown on nonpolar and semipolar orientations due to anisotropic band structures has been theoretically predicted and experimentally demonstrated [8-9]. Also, nonpolar m-plane LDs have a higher slope efficiency than c-plane LDs under actual LD operation [10-12]. However, the longest lasing wavelengths for m-plane LDs obtained by the present invention's research group is 492 nm [13], by using miscut m-plane GaN substrates [14], and the longest published lasing wavelength on a nominally on-axis m-plane was 499.8 nm [15]. The difficulty of achieving indium incorporation in the multiple quantum wells (MQWs), and the possibility of basal plane stacking fault (BPSF) formation in the wells [16], have so far limited the lasing wavelength to less than 500 nm for m-plane LDs.
Semipolar planes of GaN offer an approach that reduces polarization-related electric fields and possibly increases gain in comparison to c-plane GaN. The semipolar plane (20-21) has demonstrated lasing wavelengths of 531 nm under pulsed operation [17], and lasing wavelengths of 523 nm under CW operation [18]. To achieve high internal quantum efficiency from the high In content quantum well (QW) emitting green light, indium segregation and defects generated due to large strains must be eliminated in the high indium content QW. Enya et al. utilized lattice-matched quaternary AlInGaN cladding to reduce strain in the epitaxial structure and realize sufficient optical confinement [17]. Tyagi et al. reported high indium content InGaN guiding layers with GaN cladding layers as another method to avoid the difficulty of quaternary AlInGaN growth [19]. Although long wavelength LDs grown on (20-21) bulk GaN substrates have been demonstrated, a detailed growth study for high quality active region growths has not been reported.
In addition, conventional (20-21)-plane LD structures include the following characteristics:
1. Conventional state-of-the-art (20-21)-plane LDs are grown with InGaN or GaN barriers in the active region, as shown in FIG. 1. FIG. 1 illustrates a LD device 100 structure comprising a (20-21) GaN substrate 102, n-type GaN (n-GaN) layer 104 on or above the GaN substrate 102, n-GaN cladding layer 106 on or above the n-GaN layer 104, n-InGaN bulk Separate Confinement Heterostructure (SCH) layer 108 (with 5-10% In composition, e.g., 6%) on or above the n-GaN cladding layer 106, a light emitting active layer 110, comprising one or more InGaN quantum wells with GaN or InGaN barriers, on or above the n-InGaN bulk SCH 108, a p-type AlGaN (p-AlGaN) electron blocking layer (EBL) 112 on or above the active layer 110, a p-InGaN bulk SCH layer 114 (with 5-10% In composition, e.g., 6%) on or above the p-AlGaN EBL 112, a p-type GaN (p-GaN) cladding layer 116 on or above the p-InGaN bulk SCH layer 114, and a p++-type GaN contact layer 118 on or above the p-GaN cladding layer 116.
2. Conventional state-of-the-art (20-21)-plane LDs do not use high In content InGaN superlattice SCH layers.
3. Conventional state-of-the-art (20-21)-plane LDs do not use asymmetric InGaN/GaN short period superlattice (SPSLS) for the InGaN SCH layers.
Consequently, there is a need in the art for improved LD structures. The present invention satisfies that need. The present invention discloses a high quality active region growth with AlGaN barrier (e.g., AlGaN/InGaN MQWs) and demonstrates 516 nm lasing emission under room temperature for semipolar (20-21) nitride.